Serogroup characterisation, co-pathogen causality, and environmental surveillance gaps in a cross-sectional study of Legionella pneumophila in hospital-acquired pneumonia

Dear Editor, We read with great interest the cross-sectional study by Sivalekshmi et al. reporting a 12.5% prevalence of Legionella pneumophila (Lp) in microbiologically confirmed hospital-acquired pneumonia (HAP) cases from Eastern India using 16S rRNA gene PCR, published in Lung India.[1] The study’s finding that culture and urinary antigen testing (UAT) detected no Lp cases in a cohort where PCR identified 20- underscoring the diagnostic gap of conventional methods for non-serogroup 1 Lp in HAP − is an important sentinel observation for Indian hospitals. We wish, however, to raise three concerns that affect the strength and interpretation of the conclusions. The primary concern is the basis for attributing all PCR-positive cases to non-serogroup 1 Lp. The commercially available PCR kit employed amplifies a 901 bp region of the 16S rRNA gene specific to L. pneumophila broadly, without serogroup discrimination.[2] The inference that all 20 cases represent non-serogroup 1 strains rests entirely on UAT negativity − but UAT negativity may alternatively reflect insufficient urine antigen concentration, early-phase testing, prior antimicrobial administration, or non-sg1 strains with reduced LPS expression, as the authors themselves acknowledge. Serogroup confirmation requires either serogroup-specific PCR, mip gene sequencing, or immunological characterisation. Without this, the designation ‘non-serogroup 1’ in the title and conclusion is inferential rather than definitively established, which is an important distinction for epidemiological tracing and infection control.[3] The second concern relates to causality attribution in co-pathogen cases. Fifteen of 20 Lp-positive HAP episodes (75%) involved concurrent isolation of other gram-negative organisms. The authors acknowledge that distinguishing sequential from concurrent or incidental Lp detection was not feasible. In mechanically ventilated patients receiving broad-spectrum antibiotics, PCR detection of Lp from respiratory samples may reflect airway colonisation or incidental carriage rather than primary pneumonitis in a subset of cases.[4] The mortality difference between Lp-positive and Lp-negative HAP (55% vs. 37.1%, P = 0.133) did not reach statistical significance, which limits the strength of conclusions regarding Lp as an independent driver of clinical severity in this cohort. Third, the absence of Lp from all 36 environmental water samples collected over 17 months − despite 20 clinical cases identified − represents an important epidemiological gap. Reactive, event-triggered environmental sampling may miss intermittent or biofilm-associated contamination; Legionella species are well-known to persist in plumbing biofilms at concentrations below the detection threshold of standard plate culture.[5] A prospective, protocol-driven water surveillance programme with systematic biofilm sampling at high-risk sites, independent of clinical case occurrence, would be necessary to definitively characterise the nosocomial exposure source. These observations do not diminish the study’s valuable contribution in establishing Lp as a diagnostically overlooked pathogen in Indian HAP. We hope the authors will address these points in a response, and we encourage prospective multicentre studies with serogroup-confirmed molecular testing, structured environmental surveillance, and standardised clinical attribution criteria to consolidate these sentinel findings. Financial support and sponsorship Nil. Conflicts of interest There are no conflicts of interest.